<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>封装ajax</title>
</head>
<body>

</body>
<script>
function ajax(url){
    return new Promise((resolve,reject)=>{
        let xhr = new XMLHttpRequest()
        xhr.open('get',url) // 链接
        xhr.send() // 发送资源
        xhr.onreadystatechange = function(){ // 进行状态监听
            console.log(xhr);
            if(xhr.readyState == 4){
                if(xhr.status >= 200 && xhr.status < 300){
                    let res = JSON.parse(xhr.responseText) //json 字符串
                    resolve(res) //json 字符串
                }else{
                    reject(xhr.responseText)
                }
            }
        }
    })
}

ajax('./01.json')
.then((res)=>{
    console.log(res);
})
.catch((err)=>{
    console.log(err);
})
</script>
</html>